Observations of conduction driven evaporation in the early rise phase of solar flares

TL;DR

This study investigates pre-flare soft X-ray emissions in solar flares, suggesting heat conduction-driven chromospheric evaporation as an alternative to the classical particle beam model, based on detailed RHESSI observations.

Contribution

It provides the first detailed analysis of the pre-flare phase, demonstrating heat conduction as a plausible energy transport mechanism in early flare development.

Findings

Pre-flare emission is purely thermal with increasing emission measure.

Non-thermal tail appears later, still confined to the corona.

Material is added to the corona via chromospheric evaporation.

Abstract

In the classical flare picture, hard X-ray emission from the chromosphere is succeeded by soft-X-ray emission from hot plasma in the flare loop, the soft X-ray emission being a direct consequence of the impact of the non-thermal particle beam. However, observations of events exist in which a pronounced increase in soft X-ray emission is observed minutes before the onset of the hard X-ray emission. Such pre-flare emission clearly contradicts the classical flare picture. For the first time, the pre-flare phase of such solar flares is studied in detail. We want to explain the time evolution of the observed emission by means of alternative energy transport mechanisms such as heat conduction. RHESSI events displaying pronounced pre-flare emission were analyzed in imaging and spectroscopy. The pre-flare phase is characterized by purely thermal emission from a coronal source with increasing emission measure and density. After this earliest phase, a small non-thermal tail to higher energies appears in the spectra, becoming more and more pronounced. However, images still only display one X-ray source, implying that this non-thermal emission is coronal. The increase of emission measure and density indicates that material is added to the coronal region. The most plausible origin is evaporated material from the chromosphere. Energy provided by a heat flux is capable of driving chromospheric evaporation.